First I created my photo strip and arranged my papers. Then I selected my ribbons and arranged them with the curly ribbon on top of the blue ribbon.

Then I copied the the curly ribbon layer (Ctrl/Cmd-J) and dragged one copy below the blue ribbon. See the two highlighted layers in my layers palette? Those are the two copies of my ribbon.

Next, I gave the bottom curly ribbon shadowing. I will not give a shadow to the top copy. I want the shadow on the bottom layer. If I shadowed the top layer, when I erased parts of it, the shadows would look wrong.

Then I selected the blue ribbon pixels, not the whole layer, by holding down Control/Command and clicking on the thumbnail of the blue ribbon in the layers palette. You must click on the thumbnail only. This creates the marching selection ants around the blue ribbon, as you can see in the next screenshot, indicating that whatever I do next will only happen within that selected area.

After making that selection, I clicked on the top curly ribbon layer and selected the eraser tool. I want to erase the parts of the curly ribbon that I intend to be hidden by the blue ribbon. Here's my erasing in progress.

Notice in the screenshot that I have erased a portion of the curly ribbon on the far left and I'm almost done erasing a portion in the middle. I used a hard round brush and the bracket keys to decrease/increase size of the brush as needed. I was careful near the curls so I wouldn't erase parts I want to be in front of the blue ribbon. To deselect, hit Ctrl/Cmd-D.

That does it! After that, I added my other embellishments and finished my layout. If you try this technique, post a link in the comments so we can see your layout. Happy scrapping!